1. Field of the Invention
The present invention relates to transfer paper used in a color copying machine, in particular, a color electrophotographic apparatus having a double-side output capability, and to a method of forming color images by using the transfer paper.
2. Description of the Related Art
In recent years, an apparatus having full-color output capability and comprising a reader section R for reading a manuscript and a printer section P for outputting images in accordance with manuscript read signals, as shown in FIG. 1, is generally a type of laser printer. An image formation sequence of the apparatus in a case of a full-color mode will be explained briefly. Referring to FIG. 1, when a photosensitive drum 2 is rotated in the direction of an arrow B, a photosensitive member on the photosensitive drum 2 is uniformly charged by a charger 3. Next, image exposure is performed by a laser light E modulated in accordance with a yellow image signal of the color separation signals of a manuscript read by the reader section R, and an electrostatic latent image is thus formed. Next, as a rotor 4a rotates, the latent image is developed by a yellow developer 4Y which is moved to and fixed beforehand at a development position.
Next, a transfer member housed within a cassette 101 or 102 is taken out of the cassette by a paper feeding roller 103 or 104, respectively. Further, the transfer member which has been passed through a paper feeding guide 4A, a paper feeding roller 106, and then a paper feeding guide 4B, is held by a gripper 6 at a predetermined timing, and then electrostatically wound around a transfer drum 8 by an abutment roller 7 and an electrode facing the abutment roller 7. The transfer drum 8 is rotated in the direction of an arrow A in synchronization with the photosensitive drum 2. The visual image developed by the yellow developer 4Y is transferred by a transfer charger 9 at a place where the outer peripheral surface of the photosensitive drum 2 abuts the outer peripheral surface of the transfer drum 8. The transfer drum 8 continues to rotate as it is so as to be ready for the transferring of the next color (magenta in FIG. 1).
The electric charge of the photosensitive drum 2 is eliminated by a charger 10 for eliminating electric charge. After the photosensitive drum 2 is cleaned by cleaning means 11, it is charged again by a primary charger 3 and undergoes such image exposure as described above in accordance with the next magenta image signal. A rotary developing apparatus then rotates while an electrostatic latent image is formed on the photosensitive drum 2 in accordance with a magenta image signal as a result of the above-mentioned image exposure in order to position a magenta developer 4M at the above-mentioned predetermined development position and perform a predetermined magenta development. Then, the above-described process is performed for cyan and black colors. When the transfer of four colors has been completed, the electric charge of the four-color visual image formed on the transfer member is eliminated by the charger 10 and 13. Then, the transfer member is released by the gripper 6, is separated from the transfer drum 8 by a separation claw 14 and the transfer member is sent to a fixer 16 by a transport belt 15. The transfer member is then fixed by heat and pressure in the nip spacing formed by a fixing roller 161 whose surface is heated by a heating roller 163 and formed by a pressure roller 162, and then the transfer member is ejected onto a tray 17, thus completing a series of full-color print sequences. After the fixing roller 161 has finished fixing the transfer member, the roller is cleaned by a cleaning web 164 so as to be prepared for the next fixing operation.
In forming color images using chromatic color toners based on such an electrophotographic method to reproduce a great number of colors, as described above, toners which are coloring powder of yellow, magenta, cyan and black are stacked on the transfer member in multiple layers, and a toner resin is melted by fixing so as to be mixed, that is, color mixing, thereby achieving the above purpose. For this reason, unlike printing using printing ink, a considerable amount of coloring pigment is placed on the paper which is a transfer member, thus causing the external-light shielding power to increase. Under such conditions, as regards a transfer member which can be used in a conventional full-color electrophotographic apparatus as paper formed from chemical pulp, there has been a demand that the transfer member has a proper degree of whiteness which serves as a base for color reproduction, is able to provide a low thermal capacity in which the above-mentioned color mixture by heating is possible, and has a volume resistivity in which electrostatic transferring of at least three times is possible, and having sufficient flexibility to allow the transfer member to be electrostatically wound around the transfer drum. These conditions may be satisfied by using plain paper, which paper generally has a whiteness degree of 85% or more, opaqueness degree of 85% or less, a volume resistivity of 1.times.10.sup.10 to 10.times.10.sup.11 .OMEGA..multidot.cm (20.degree. C., 65%), a stiffness of 17 to 22 cm (JIS P-8143 A process) though the plain paper has a weight slightly greater than that of ordinary black and white electrophotographic paper. The value of stiffness is measured in the following way. The length from the grasping portion to the leading edge of a test piece when the direction in which the leading edge of the test piece is hung and bent becomes an opposite direction if it is rotated 90.degree. with the grasp line as an axis when one end of the test piece having a long thin, fixed shape is grasped and held upward, is the value of the stiffness.
However, in a case in which a full-color image of only one surface is formed using the apparatus shown in FIG. 1 by use of the above-mentioned conventionally used paper, when a full-color image is formed nevertheless on both sides of the paper, there is the possibility that a color tone of the image on a second surface will be considerably affected depending upon the presence or absence of toner on the first surface of the paper when the paper is raised by the hiding power of the toner. Further, since the color of the first surface is made visible in the form of a watermark on a white portion on the second surface even if the first surface is a single image having a uniform density, chroma from a halftone portion to a highlight portion deteriorates considerably. Further, since the toner on the first surface is melted because fixing is performed twice, the toner deeply penetrates the paper, causing offset to increase.